The Blackhawks have seven days to match the offer. If they choose not to match, they will receiver San Jose's first- and third-round picks in next year's NHL draft as compensation.

The 23-year-old Hjalmarsson would help replace captain Rob Blake, who announced his retirement at age 40 last month. The Sharks were looking for a defenceman on the market and found one from one of their key competitors in the Western Conference.

"We feel Niklas is a top-three defenceman in the National Hockey League," general manager Doug Wilson said. "We saw his abilities firsthand in the playoffs last season and he was an important piece of a Stanley Cup-winning team. He is a solid player that would be a good fit on our team now and in the future."

Hjalmarsson helped Chicago sweep the Sharks in the Western Conference final last season and then beat Philadelphia to win the Stanley Cup. He had a goal and seven assists in the post-season, including two assists against the Sharks.

He was third on the team with a plus-9 rating in the playoffs, while averaging just over 21 minutes of ice time per game.

The Blackhawks were facing salary cap problems after winning the Cup and have already lost defenceman Brent Sopel and key forwards Dustin Byfuglien, Kris Versteeg and Andrew Ladd this off-season. They now have a decision to make on what to do with Hjalmarsson.

He had two goals and 15 assists in 77 regular season games. That was his first full season in the NHL, having played sparingly for the Blackhawks the previous two years. He has three goals and 18 assists in 111 career games.

If Hjalmarsson does join the Sharks, he would be the second key addition in free agency this off-season, following last week's signing of goalie Antero Niittymaki.